Portable batteries with increased energy and power densities are required as the use of portable electronic equipment rapidly continues to increase. Batteries are typically the limiting factor in the performance of most portable commercial and military electronic equipment due to the restrictions on the size, weight and configuration imposed on the equipment by limitations from the power source. In some cases, safety and environmental factors are also significant considerations for deploying a particular power source. Lithium batteries provide high energy density, conformal packaging and improved safety, which make them one of the most promising electrochemical systems under development today.
Lithium batteries use high valence metal oxide materials, which are reduced during the electrochemical reaction. This reaction in rechargeable lithium and rechargeable lithium ion batteries must be fully reversible in order to have a commercially viable cell. Common reversible metal oxide materials used in lithium batteries include: LixMn2O4, LixMnO2, LixCoO2, LixNiO2 and LixNiyCozO2. These materials remain reversible against lithium whenever lithium subscript “x” is maintained between 0.10 and 0.85 for LixMn2O4, 0.1 and 0.5 for LixMnO2 and 0.4 and 0.95 for LixCoO2, LixNiO2 and LixNiyCozO2. However, if the stoichiometry exceeds these limitations, the material undergoes a phase change and is no longer reversible. The primary consequences of the phase change of the material and subsequent irreversibility are that the cell will no longer accept a charge, which makes the cell inoperable. In order to maintain this stoichiometry rigid electronic control is usually employed, but rigid controls such as current and voltage limiters employed at the stack level or cell level are not practical for many situations where lithium batteries are deployed, which makes maintaining reversibility even more critical for lithium electrochemical systems used in portable electronic equipment.
Thus there has been a long-felt need to solve the problems associated with maintaining reversibility in lithium batteries without suffering from the disadvantages, limitations and shortcomings associated with rigid stoichiometry electronic control and phase change. A mixed metal oxide that introduces bismuth into the manganese oxide cathode structure yields a material with reduced charge transfer impedance due to catalytic activity. This reduced charge transfer impedance provides a lower potential charge mechanism avoiding the problems associated with loss of reversibility in lithium batteries without suffering from the disadvantages, limitations and shortcomings associated with rigid stoichiometry electronic control and phase change.